The present invention relates to a ball end mill of the throw-away type, capable of uniformizing wear on the cutting edges of the tips.
For a ball end mill having a diameter relatively large, on the order of 30 mm to 60 mm, generally, a throw-away type is more widely used more than a solid type. FIGS. 13 through 15 illustrate an example of such a conventional ball end mill of the throw-away type (hereinafter referred to simply as "ball end mill"). An end mill body 1 has its forward end to which a pair of throw-away tips (hereinafter referred to simply as "tips") 2 and 2 are removably mounted. Each tip 2 is generally in the form of a triangle, and has an upper surface whose ridgelines are formed respectively into convexly arcuate cutting edges 2a. Each cutting edge 2a has an end which overlaps with an end of the adjacent cutting edge 2a in a rotational locus of the radially outward cutting edges 2a of the respective tips 2 and 2, to form substantially a 1/4 arc of the rotational locus.
For this ball end mill, the overlapping length l of the cutting edges 2a and 2a is short, being on the order of 1 mm to 2 mm in the arcuate length of the rotational locus. Accordingly, wear on each cutting edge 2a is severe. For this reason, the conventional ball end mill has such a drawback that it cannot cope with high output processing.
A ball end mill shown in FIGS. 16 through 20 has been proposed, which can solve the above drawback. The ball end mill illustrated in these figures has a pair of tips 3 and 4 which are mounted to an end mill body 1. Each of the tips 3 and 4 is substantially in the form of an elliptical configuration. An entire periphery of an upper surface of each of the cutting edges 3 and 4 is formed into a pair of cutting edges 3a and 3a, 4a and 4a. An overlapping length l between the cutting edges 3a and 4a is set large.
However, the above ball end mill has the following various drawbacks:
(1) The tips 3 and 4 increase in their outer dimension extremely, which results in an increase in their cost.
(2) The contact length of the tips 3 and 4 with a workpiece increases, so the cutting resistance increases.
(3) Since the tips 3 and 4 are large in size, a pair of tip mounting seats 5 and 5 formed in the end mill body 1 must inevitably increase in size in order to cause the tips 3 and 4 to rest respectively on the tip mounting seats 5 and 5. Thus, the tool rigidity of the end mill body 1 decreases.